1. Field of the Invention
This invention relates to a novel and improved accelerator for charged particles. In particular, this invention relates to an accelerator which forms a narrow beam of accelerated particles in an accelerator chamber and which includes a discharge window arranged on said chamber for receiving said beam of particles at a striking location and for discharging the particles therethrough. Still more particularly, this invention relates to an electron accelerator, preferably to a linear accelerator, the accelerator chamber of which accelerates electrons until high energies are reached.
A linear accelerator can be used in a number of different applications in the field of medical treatment, such as radio therapy, radiography, and sterilization. The irradiation treatment may be carried out either by employing the accelerated electrons of high energy, or by gamma rays (hard X-rays) generated by the accelerated electrons after hitting a target.
2. Description of the Prior Art
The term "charged particle accelerator" used herein shall particularly comprise all kinds of electron accelerators that emit accelerated electrons through an electron exit window, such as betatrons and linear accelerators.
In a known type of a linear accelerator for medical purposes, an electron beam emitted from an electron source is directed in an acceleration chamber towards an electron exit window. The electron beam is pulsed and sharply focused. In the process of acceleration, the electrons attain a relatively high energy. For instance, as electron source may be used an electron gun having a tungsten cathode. The electron exit window is usually made of a thin metal foil. At the electron exit window, the electrons may have an energy of, for example, 4 MeV.
Linear accelerators of this type are, as already mentioned, mainly used for medical purposes. For example, the high energy electron beam discharged from the electron exit window may be directly employed to irradiate pathological tissue of a patient, or the high energy electron beam may be directed onto a target where it generates gamma rays (X-rays of high energy) which are applied for therapeutical treatment.
The particle exit or discharge window of particle accelerators pose special problems. That is particularly true for the electron exit window of an electron accelerator, particularly of a linear accelerator.
On the one hand, the exit window is exposed to a heavy thermal stress and strain. The impinging particle beam of high energy, in a linear accelerator the beam of accelerated electrons, has only a small diameter so that the striking location--that is, the place where the beam of particles hits the discharge window--is also small. In linear accelerators, the striking location typically may have a diameter of about 0.5 mm. Heat generated by the accelerated electrons is concentrated and the exit window may be locally overheated at the striking location. Even when water-cooling is employed, it may happen that the discharge window burns through, so that the vacuum in the interior of the particle accelerator is destroyed.
On the other hand, secondary electrons are generated at the discharge window and released into the interior of the accelerator. This is particularly true for the electron exit window of an electron accelerator. Secondary electrons have only thermal energy. However, in a linear accelerator having, for instance, a standing wave guide as the accelerator chamber or accelerator tube, secondary electrons may be accelerated in the backward direction along the central axis of the accelerator tube towards the electron source. At the electron source, they may arrive with the same high energy, to which the forward-directed electrons are accelerated, e.g. about 4 MeV. Under the effect of these high energy electrons, the electron source, for instance, the above-mentioned tungsten cathode, will emit gamma radiation (X-rays of high energy). This gamma radiation, the so-called leakage radiation, will also be directed backwards. The operator(s) of the particle accelerator must clearly be protected from this leakage radiation. This makes necessary special screening devices, which usually require some additional expenditure. The costs are high and sufficient space is sometimes unavailable.